"At 20% of the speed of light even a small amount of tenous gas or dust will turn the craft into high temperature plasma."
The Local Interstellar Cloud contains both Sol and Alpha Centauri and is about 0.3 particles/cc. Iff the energy from all those hits was fully absorbed, it would result in a few millijoules per second of heating. (I've assumed the atoms were stationary -- i.e. ignored their thermal energy.)
But heating is not going to be a problem because those particles are protons with tens of MeV: they will punch through the wafer-thin probe, taking most of their energy with them. The sail is expendable, once accelerated. But the electronics needs to survive a billion cosmic rays per second for decades. Expect to find more of your electronics on the outside that the inside. As I said, evaginated.